R. Reshef

Dynamic microglial alterations underlie stress-induced depressive-like behavior and suppressed neurogenesis.

The limited success in understanding the pathophysiology of major depression may result from excessive focus on the dysfunctioning of neurons, as compared with other types of brain cells. Therefore, we examined the role of dynamic alterations in microglia activation status in the development of chronic unpredictable stress (CUS)-induced depressive-like condition in rodents. We report that following an initial period (2–3 days) of stress-induced microglial proliferation and activation, some microglia underwent apoptosis, leading to reductions in their numbers within the hippocampus, but not in other brain regions, following 5 weeks of CUS exposure. At that time, microglia displayed reduced expression of activation markers as well as dystrophic morphology. Blockade of the initial stress-induced microglial activation by minocycline or by transgenic interleukin-1 receptor antagonist overexpression rescued the subsequent microglial apoptosis and decline, as well as the CUS-induced depressive-like behavior and suppressed neurogenesis. Similarly, the antidepressant drug imipramine blocked the initial stress-induced microglial activation as well as the CUS-induced microglial decline and depressive-like behavior. Treatment of CUS-exposed mice with either endotoxin, macrophage colony-stimulating factor or granulocyte-macrophage colony-stimulating factor, all of which stimulated hippocampal microglial proliferation, partially or completely reversed the depressive-like behavior and dramatically increased hippocampal neurogenesis, whereas treatment with imipramine or minocycline had minimal or no anti-depressive effects, respectively, in these mice. These findings provide direct causal evidence that disturbances in microglial functioning has an etiological role in chronic stress-induced depression, suggesting that microglia stimulators could serve as fast-acting anti-depressants in some forms of depressive and stress-related conditions.

Depression as a Microglial Disease

Despite decades of intensive research, the biological mechanisms that causally underlie depression are still unclear, and therefore the development of novel effective antidepressant treatments is hindered. Recent studies indicate that impairment of the normal structure and function of microglia, caused by either intense inflammatory activation (e.g., following infections, trauma, stroke, short-term stress, autoimmune or neurodegenerative diseases) or by decline and senescence of these cells (e.g., during aging, Alzheimers disease, or chronic unpredictable stress exposure), can lead to depression and associated impairments in neuroplasticity and neurogenesis. Accordingly, some forms of depression can be considered as a microglial disease (microgliopathy), which should be treated by a personalized medical approach using microglial inhibitors or stimulators depending on the microglial status of the depressed patient.

Microglia and their CX3CR1 signaling are involved in hippocampal- but not olfactory bulb-related memory and neurogenesis.

Recent studies demonstrate that microglia play an important role in cognitive and neuroplasticity processes, at least partly via microglial CX3C receptor 1 (CX3CR1) signaling. Furthermore, microglia are responsive to environmental enrichment (EE), which modulates learning, memory and neurogenesis. In the present study we examined the role of microglial CX3CR1 signaling in hippocampal- and olfactory-bulb (OB)-related memory and neurogenesis in homozygous mice with microglia-specific transgenic expression of GFP under the CX3CR1 promoter (CX3CR1(-/-) mice), in which the CX3CR1 gene is functionally deleted, as well as heterozygous CX3CR1(+/-) and WT controls. We report that the CX3CR1-deficient mice displayed better hippocampal-dependent memory functioning and olfactory recognition, along with increased number and soma size of hippocampal microglia, suggestive of mild activation status, but no changes in OB microglia. A similar increase in hippocampal-dependent memory functioning and microglia number was also induced by pharmacological inhibition of CX3CR1 signaling, using chronic (2weeks) i.c.v. administration of CX3CR1 blocking antibody. In control mice, EE improved hippocampal-dependent memory and neurogenesis, and increased hippocampal microglia number and soma size, whereas odor enrichment (OE) improved olfactory recognition and OB neurogenesis without changing OB microglia status. In CX3CR1-deficient mice, EE and OE did not produce any further improvement in memory functioning or neurogenesis and had no effect on microglial status. These results support the notion that in the hippocampus microglia and their interactions with neurons via the CX3CR1 play an important role in memory functioning and neurogenesis, whereas in the OB microglia do not seem to be involved in these processes.

The hippocampal transcriptomic signature of stress resilience in mice with microglial fractalkine receptor (CX3CR1) deficiency

Clinical studies suggest that key genetic factors involved in stress resilience are related to the innate immune system. In the brain, this system includes microglia cells, which play a major role in stress responsiveness. Consistently, mice with deletion of the CX3CR1 gene (CX3CR1-/- mice), which in the brain is expressed exclusively by microglia, exhibit resilience to chronic stress. Here, we compared the emotional, cognitive, neurogenic and microglial responses to chronic unpredictable stress (CUS) between CX3CR1-/- and wild type (WT) mice. This was followed by hippocampal whole transcriptome (RNA-seq) analysis. We found that following CUS exposure, WT mice displayed reduced sucrose preference, impaired novel object recognition memory, and reduced neurogenesis, whereas CX3CR1-/- mice were completely resistant to these effects of CUS. CX3CR1-/- mice were also resilient to the memory-suppressive effect of a short period of unpredictable stress. Microglial somas were larger in CX3CR1-/- than in WT, but in both genotypes CUS induced a similar decline in hippocampal microglial density and processes length. RNA sequencing and pathway analysis revealed basal strain differences, particularly reduced expression of interferon (IFN)-regulated and MHC class I gene transcripts in CX3CR1-/- mice. Furthermore, while CUS exposure similarly altered neuronal gene transcripts (e.g. Arc, Npas4) in both strains, transcripts downstream of hippocampal estrogen receptor signaling (particularly Igf2 and Igfbp2) were altered only in CX3CR1-/- mice. These findings indicate that emotional and cognitive stress resilience involves CX3CR1-dependent basal and stress-induced alterations in hippocampal transcription, implicating inhibition of CX3CR1 signaling as a novel approach for promoting stress resilience.

Abstract # 1756 The role of microglia in adult neurogenesis

Microglia are known to play important roles in brain development and plasticity, but their roles in development and maturation of synapses on adult-born neurons remains unknown. To elucidate the role of microglia in neurogenesis, we labeled newborn granule neurons (GNs) migrating to the olfactory bulb (OB) in microglia-depleted mice (induced by CSF-1 receptor inhibition) and CX3CR1 deficient (CX3CR1-/-) mice, by injecting a tdTomato-expressing lentivirus. 28 days later we imaged infected maturing OB GNs using 2-photon microscopy, followed by immunohistochemical examination of their spine density and size. Our data shows that microglia-depleted and CX3CR1-/- mice had a significant reduction in spine density. Moreover, CX3CR1-/- mice had smaller spine heads. Time-lapse data indicated that microglia depletion and CX3CR1 deficiency caused stabilization of synaptic turnover in maturing GNs, as fewer dendritic spines were formed and lost over 24h. These morphological changes were found to have functional significance as in vivo calcium imaging showed enhanced responses to odors of mitral cells, which are normally inhibited by GNs. RNA-Seq analysis indicated that the morphological changes in the two experimental models were associated with different molecular mechanisms, including the complement and cytokines transcriptional programs in microglial depletion, and MHC-I, wnt and interferon transcriptional programs in CX3CR1 deficiency. Our findings indicate that microglia are involved in normal development, maturation and plasticity of adult-born OB neurons, with functional implications to the OB output.

Effects of chronic stress-induced depression in mice on the number and morphology of microglia

250 Sub-septic systemic inflammation does not impair performance in social cognition tasks J.S. Grigoleit , J.S. Kullmann , O.T. Wolf , H. Engler , E.R. Gizewski , J.R. Oberbeck , M. Schedlowski a a Institute of Medical Psychology and Behavioral Immunobiology, Medical Faculty, University of Duisburg-Essen, Hufelandstrasse 55, Essen 45122, Germany b Department of Cognitive Psychology, Ruhr-University Bochum, Germany c Institute for Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Germany d Clinic for Trauma Surgery, University Hospital Essen, Germany Clinical and experimental evidence document, that increased cytokine plasma levels evoked by cytokine therapy, chronic diseases or acute experimental immune activation with lipopolysaccharide (LPS) are associated with depression-like symptoms and neuropsychological disturbances in humans. In contrast, the effects of increased peripheral cytokine levels on emotional and social cognition are largely unknown. Thus, we employed a human endotoxemia model to analyze the impact of low-grade inflammation on social cognitive performance. In a randomized, placebo-controlled cross-over-designed study 18 male volunteers received either LPS (0.4 ng/kg) or NaCl. Body temperature, and plasma levels of cortisol, Interleukin(IL)-6, IL-10, IL1ra and Tumor Necrosis Factor(TNF)-alpha were analyzed before and 1, 2, 3, 4, 6 and 24 h after injection. In addition, the ability to assess facial expressions (‘‘Reading the Mind in the Eyes”) and social cognition performance (‘‘Movie for the Assessment of Social Cognition”/ MASC) was analyzed. LPS administration induced a clear-cut immune activation characterized by significant increases in body temperature, plasma cytokine and cortisol levels. In addition, volunteers reported increased fatigue, feelings of anxiety and impaired mood during low grade inflammation. In contrast, LPS-induced cytokine increases did neither affect performance in the facial expression task nor in the MASC. Theses data indicate that increased peripheral cytokine levels affect mood and anxiety but are not associated with alterations in social or emotional cognition. doi:10.1016/j.bbi.2010.07.044